If you are using afterburner's Kombustor stress test, it only uses one card and would cause the usage you are seeing.
Also, not all games are optimized to use two cards and thus may just throw everything onto one of the two cards.
As for the temp difference, It's common in SLI setups. I have a complete slot space between my two GTX 560's and the top one genrally runs ~5 to 10C hotter than the bottom one. I even have two 120mm fans driving air straight to them. I have a slight overclock with no extra voltage and my max temp for the hot card is ~70C
The other one maxes out around 65C. But it's not sucking air off the back of a vido card the way the top one is.